[xsl] (Possible) pitfall: XSLT 2, 9.4 Creating implicit document nodes

Hello,

this is a heads-up for a pitfall (that at least I have fallen into several 
times now...) with respect to XSLT 2, 9.4 "Creating implicit document nodes":

<http://www.w3.org/TR/xslt20/#temporary-trees>

With the source document:

--src.xml--
<a>
  <b/>
</a>
-----------

and this transformation

--transformation.xsl--
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
  xmlns:xs="http://www.w3.org/2001/XMLSchema";
  exclude-result-prefixes="xs"
  version="2.0">
  
  <xsl:template match="/">
    <xsl:variable name="v1"><xsl:sequence select="a"/></xsl:variable>
    <xsl:variable name="v2" select="a"/>

    
    <xsl:text>v1: </xsl:text>
    <xsl:apply-templates select="$v1/a" mode="out"/>
    <xsl:text>&#xa;</xsl:text>
    
    <xsl:text>v2: </xsl:text>
    <xsl:apply-templates select="$v2/a" mode="out"/>
  </xsl:template>
  
  
  <xsl:template match="*" mode="out">
    <xsl:value-of select="name()"/>
    <xsl:apply-templates mode="#current"/>
  </xsl:template>
  
  
  <xsl:template match="text()" mode="#all"/>
</xsl:stylesheet>
-----------

the output is:

--output--
v1: ab
v2: 
----------

I understand now why this is so per the spec, although I was puzzled at first.

However, I've spent considerable time debugging stylesheets where accessing a 
tunnel variable with $var/elem sometimes yielded nothing (see v2 in my example 
above) until I found that in some places, the variable was defined using 
<xsl:sequence> (because some complex content construction takes place which is 
either not doable or hardly readable using a single XPath expression), and 
using a select attribute at other places.

Is there a technique or pattern I could employ (maybe utilizing the @as 
attribute somehow?) to unify the access to variable contents where I know that 
the sequences are node sequences, regardless of their content construction 
using @select or <xsl:sequence>?

Thanks,
Christian
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